Large off-highway haul vehicles, such as those used in open pit mines, quarries, etc., typically use outboard mounted, planetary drives as part of drive axle assembly. These outboard planetary drives act as gear reducers between the drive axle and the tire hub. The planetary unit reduces the rotational speed of the hub relative to the drive axle speed and acts as torque multiplier.
Since substantial torques are transmitted through the planetary unit, they are typically constructed of robust components. In at least some cases, spur gears are used for the sun gear and planet gears. Helical gearing is also used for some units. It has been found, that significant noise levels are produced by the planetary units during vehicle operation. This noise may be generated by the gears themselves, and/or vibrations generated in the units. Since the planet gears are carried by a planet carrier which typically forms a part of the planetary cover, vibrations generated in the planetary are transmitted to the cover which in affect act as a speaker or drum head and converts vibrations to noise. It has been found that the noise generated by the planetary units of the these types of vehicles can be objectionable especially in open pit mines near communities.
The noise generated by the hub reduction gearing comes from two discrete sources. The first noise is generated by the gear teeth coming into and out of mesh. This sliding action generates noise. The amount of noise generated by the mesh itself depends on the tooth profile, pressure angle, and quality class of gear, and accuracy of the housing. The second source of noise is due to the stiffness of the housing or carrier. Since off-highway equipment demands very high tractive effort, the carriers must be made from steel or ductile iron. The carriers must also be very rigid to effectively keep the gear teeth aligned properly. The stiffness combined with the material properties place the natural frequency of the planet carrier within the operating frequency of the gear reduction. When the geartrain operates at or near the natural frequency of the carrier, high sound pressure is the result. The natural frequency of the carrier can, in theory be changed by changing the stiffness of the part. However as a practical matter there are not viable alternate materials (having a stiffness different from steel or iron) from which to construct the carrier that can withstand the forces and torques generated during the operation of the vehicle